Treating inherently aging chromium-nickel stainless steel



Patented Apr. 24, 1945 TREATING INHEBENTLY AGING CHRO- -NICKEL STAINLESS STEEL George V.

Smith, Millburn, *N. J., assignor to United States Steel Corporation of Delaware, a corporation of Delaware No Drawing. Application July 16, 1943,

Serial No. 495,029

1 Claim. (Cl. 148- 2L5) This invention is a method-of treating chro mium-nickel stainless and at least One of the steel containing carbon stronger-than-chromium carbideformers, such as titanium, columbium, zirconium, etc., with its components that are ferrite formers proportioned to those that are austenlte formers to cause the stress-laden ferrite at low temperatures steel cools to room temperature from tures where it is completely austenitic. the steel contains aluminum.

As disclosed and claimed in the Wyche and Smith application, filed October 8, 1941, Serial No. 414,194, the stress-laden ferrite, of such a steel, is inherently aging to a much higher hardness and strength. By varying the proportion between the ferrite formersand the austenite formers the steel may be made to contain more or less of this aging ferrite, with the balance austenite, but once the steel is made this is, naturally, impossible.

The present invention is a method of treating such steel so as to decrease the amount of aging ferrite while increasing the nonaging austenite. at the time of aging, by heat treatment, as contrasted to varying the composition of the steel.

According to this invention, steel, of the character described, that has been produced'in the normal fashion, amount of stress-laden ferrite its composition permits, is heated to transform at least some of this ferrite to austenite, and is then cooled to temperatures where this austenite, produced from the aging ferrite, transforms back to the ferrite with time, the steel being then held until a selected and determined prop rtion of the aging ferrite is produced, whereupon the steel is reheated to temperatures preventing further transformation of this austenite, which would otherwise transform to stress-laden ferrite, which tem-. peratures are adequate to age the ferrite that did transform, the steel being .held to at least partially age this ferrite, and then being cooled to room temperature. During the last step some, or all, of the austenite, formed from the stressladen ferrite, ferrite, but since no aging step is given thisproportion of the ferrite, it does notacquire the higher strength and hardness as has that portion when the tempera- Usually production of so as to obtain the maximum er-atures when the austenitic, said method comprising heating the may transform to the stress-laden the steel to room temperature.

that was aged. It follows that, by proper selection of time and temperature, it is possible to selectively age any given proportion of the a ng ferrite so as to, selectively, impart different properties to the steel.

Depending on the exact composition of the steel, temperatures of 1100 to 2200 F. will serve to austenitize the stress-laden ferrite, inherently possessed by the steel, duced retransforming to the stress-laden ferrite at temperatures ranging from 200 downwardly to less than room temperature, the transformation requiring time, which is limited in accordance with the amount of the normally available aging ferrite it is desired to age, while temperatures of from 700 to 1100 F. will serve to completely stop this transformation to the ferrite and to age the ferrite that was allowed to transform. As previously explained, when the steel is finally permitted to cool, develop all of its stress-laden ferrite, aflorded by its composition, but this ferrite will be unaged.

It follows from the foregoing that this invention provides an additional means foradjusting the properties of steel of the type described, other than by adjustment of its chemical composition such as would inherently lead to various amounts of stress-laden or aging ferrite.

I claim:

A method of treating chromium-nickel stainless steel containing carbon and at least one of the stronger-than-chromlum carbide formers such as titanium, columbium, zirconium, etc., with its components that are ferrite formers proportioned to those that are austenlte formers to cause the production of stress-laden ferrite at low temsteel cools to room temperature from temperatures where it is completely steel to transform at least some of said ferrite to austenite, cooling the steel to temperatures where said austenite transforms to said ferrite and holding until some but not all of said austenlte transforms to said ferrite, reheating to temperatures preventing further transformation of said austenite and which age said ferrite and holding to at least partially age said ferrite, and cooling onoaon v. sm'rn.

the austenite thus pro-- to room temperature, it may 

